A special type of teacher's password.

Eudoxia is referencing Mike Darwin's idea of modifying white blood cells with arbitrarily-sophisticated biotechnology (we're talking "you can design new organelles to spec" as a lower-level requirement) to do active cell repair, sucking up cell contents and yoinking nuclear genetic information from even very-damaged cells before digesting the old contents and replacing them. It's an elaborate thought experiment with technical-looking diagrams that elides huge black boxes in its proposed mechanism. Basically it's the idea of nanomedicine before the term was coined.